1. Field of the Invention
The present invention relates to a pneumatic tire which has a tread surface including a shoulder land portion extending in a circumferential direction of the tire, and a thin groove extending in a circumferential direction of the tire located at, as viewed in a width direction of the tire, the outer side than a contact edge of the shoulder land portion; the thin groove partitioning the shoulder land portion into a main land portion located at the inner side and a sacrifice portion located at the outer side of shoulder land portion, and the tire including a buttress groove extending in a circumferential direction of the tire located at the outer side than the contact edge of the shoulder land portion.
2. Description of the Related Art
In a pneumatic tire, generally, in an area adjacent to a contact edge of a shoulder land portion in a tread surface, the contact pressure tends to increase during running. As a result, there may be caused a problem of partial wear in which, compared to other land portion in the tread surface, an amount of wear becomes larger in the area adjacent to the contact edge of the shoulder land portion. Conventionally, a preventive method against the partial wear is widely employed, in which a thin groove extending in a circumferential direction of the tire located, as viewed in a width direction of the tire, at the inner side than the contact edge is formed in an area adjacent to the contact edge of the shoulder land portion in the tread surface. The sacrifice portion with a higher stiffness, which is partitioned by the thin groove located at the outer side of the tire, provides the higher effectiveness for reducing the partial wear. On the other hand, too high stiffness of the sacrifice portion causes stress to converge on the groove bottom of the thin groove due to the deformation of the sacrifice portion during running on the road. This causes cracks to be generated on the groove bottom of the thin groove. As a result, even in an initial to middle stage of use of the tire, the sacrifice portion may be broken.
Japanese Unexamined Patent Publication No. 2007-106255 teaches a method to reduce the partial wear in an area adjacent to the contact edge of the shoulder land portion in the tread surface. That is, a pneumatic tire has a shoulder land portion including a thin groove extending in a circumferential direction of the tire formed in an area adjacent to the contact edge, and the tire includes a buttress groove extending in a circumferential direction of the tire formed at the outer side than the contact edge of the shoulder land portion as viewed in a width direction of the tire. The thin groove is inclined inward from the tread surface toward the groove bottom as viewed in the cross-section in a meridian direction of the tire. However, in this method, since the thin groove is inclined inward from the tread surface toward the groove bottom as viewed in the cross-section in the meridian direction of the tire, the stiffness of the main land portion, which is located at the inner side than the thin groove as viewed width direction of the tire, is reduced. As a result, an amount of wear becomes larger in an area of the main land portion at the thin groove side edge (in the main land portion, in an area adjacent to an edge line formed by the thin groove and the main land portion therebetween) resulting in a problem of partial wear. Further, the method does not define the configuration of the buttress groove. Depending on the configuration of the buttress groove, cracks may occur on the buttress groove. Furthermore, in the above method, depending on the location of the buttress groove as viewed in a radial direction of the tire, the stiffness may be largely reduced in a base area of the sacrifice portion (generally the same position as that of the thin groove bottom as viewed in a radial direction of the tire). As a result, the partial wear reduction effect may be degraded.